Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and systems and, more particularly, to mechanisms and techniques for generating an acquisition scheme for vibratory sources.
Discussion of the Background
Reflection seismology is a method of geophysical exploration to determine the properties of a portion of a subsurface layer in the earth, which information is especially helpful in the oil and gas industry. Conventional reflection seismology uses a controlled source that sends mechanical waves into the earth. By measuring the time it takes for the reflections and/or refractions to come back to plural receivers, it is possible to estimate the depth and/or composition of the features causing such reflections. These features may be associated with subterranean hydrocarbon deposits.
Various sources of seismic energy have been utilized in the art to impart the seismic waves into the earth. Such sources have included two general types: 1) impulsive energy sources, and 2) seismic vibrator sources. The first type of geophysical prospecting utilizes an impulsive energy source, such as dynamite or a marine air gun, to generate the seismic signal. With an impulsive energy source, a large amount of energy is injected into the earth in a very short period of time. Accordingly, the resulting data has a relatively high signal-to-noise ratio, which facilitates subsequent data processing operations. On the other hand, use of an impulsive energy source can pose certain safety and environmental concerns.
The second type of geophysical prospecting employs a seismic vibrator (e.g., a land or marine seismic vibrator) as the energy source, wherein the seismic vibrator is commonly used to propagate energy signals over an extended period of time, as opposed to the near instantaneous energy provided by impulsive sources. Thus, a seismic vibrator may be employed as the source of seismic energy which, when energized, imparts relatively low-level energy signals into the earth. The seismic process employing such use of a seismic vibrator is sometimes referred to as “VIBROSEIS” prospecting. In general, vibroseis is commonly used in the art to refer to a method used to propagate energy signals into the earth over an extended period of time, as opposed to the near instantaneous energy provided by impulsive sources. The data recorded in this way is then correlated to convert the extended source signal into an impulse.
Typically, the impartation of energy with vibrator devices is for a preselected energization interval, and data are recorded during the energization interval and a subsequent “listening” interval. It is desirable for the vibrator to radiate varying frequencies into the earth's crust during the energization interval. In such instances, energy at a beginning frequency is first imparted into the earth, and the vibration frequency changes over the energization interval at some rate until the end frequency is reached at the end of the interval. The difference between the beginning and end frequencies of the sweep generator is known as the range of the sweep, and the length of time in which the generator has to sweep through those frequencies is known as the sweep time.
Vibrators typically employ a sweep generator, and the output of the sweep generator is coupled to the input of the vibrator device. The output of the sweep generator dictates the manner in which the frequency of the energization signal, which is imparted into the earth, varies as a function of time.
Several methods for varying the rate of change of the frequency of the sweep generator during the sweep time have been proposed. For example, in the case of a linear sweep, the frequency output of the sweep generator changes linearly over the sweep time at the rate dictated by the starting and end frequencies and the sweep time. Further, nonlinear sweeps have been proposed to shape the output frequency spectrum amplitude in which the rate of change of the frequency of the sweep generator varies nonlinearly between the starting and end frequencies over the sweep time. Examples of nonlinear sweeps have been quadratic sweeps and logarithmic sweeps.
The frequency of the seismic sweep may start low and increase with time (i.e., “an upsweep”) or it may begin high and gradually decrease (i.e., “a downsweep”). Typically, the frequency range today is, say from about 3 Hertz (Hz) to some upper limit that is often less than 200 Hz, and most commonly the range is from about 6 Hz to about 100 Hz.
The seismic data recorded during vibroseis prospecting (hereinafter referred to as “vibrator data”) comprises composite signals, each having many long, reflected wavetrains superimposed upon one another. Since these composite signals are typically many times longer than the interval between reflections, it is not possible to distinguish individual reflections from the recorded signal. However, when the seismic vibrator data is cross-correlated with the sweep signal (also known as the “reference signal”), the resulting correlated data approximates the data that would have been recorded if the source had been an impulsive energy source.
In order to increase the Vibroseis acquisition productivity, there is a present trend in the industry to perform simultaneous shooting. A problem with simultaneous shooting is the crosstalk induced by each source. This problem, usually named the “cocktail party” problem, prevents retrieving each individual shot without source-coupling noise. Several methods are investigated to treat the simultaneous shooting issues. Minimizing the mixture noise in simultaneous shooting can be achieved either after acquisition during data processing, or before acquisition by choosing adequate source signals in order to facilitate the shot separation. A possible solution is to choose orthogonal signals for driving the sources or, at least, weakly correlated signals for the sources. The design of separable orthogonal pseudo-random source signals using pseudo-random source signals as described in U.S. Pat. No. 8,274,862, assigned to the assignee of the present application, the entire content of which is incorporated by reference herein have been developed. Pseudo-random signals have good orthogonal properties. Beyond their high-productivity potential, they have the advantage of minimizing the eventual excitation of resonance frequencies of infrastructures in urban or industrial environments because their energy is spread over the overall time-frequency plan. Two difficulties noted with the use of pseudo-random source signals are: 1) they are more difficult to be handled by the vibrator's electronic servo-control mechanism than swept sine waves, and 2) the IMD (intermodulation distortion) noise can be more difficult to remove in processing than harmonic noise associated with swept sine wave signals. The first difficulty can lead to output source signals whose amplitude spectrum do not follow the amplitude spectrum of the reference signal and can vary as the source moves from shot point to shot point. The second difficulty can produce noise artifacts that if not removed can mask the reflection data.
Thus, there is a need for finding other type of orthogonal driving signals that simultaneously drive seismic sources and are easy to control with minimum intermodulation noise.